NCP1255 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1255

Current-Mode PWM Controller for Off-line Power Supplies featuring Peak Power Excursion

ON Semiconductor 

NCP1255

If we now plot the peak current set point obtained by implementing the recommended resistor values, we obtain the following

curve (Figure 7):

Peak current setpoint

100%

80%

Vbulk

375

Figure 7. The peak current regularly reduces down to 20% at 375 V dc.

The OPP pin is surrounded by Zener diodes stacked to

protect the pin against ESD pulses. These diodes accept

some peak current in the avalanche mode and are designed

to sustain a certain amount of energy. On the other side,

negative injection into these diodes (or forward bias) can

cause substrate injection which can lead to an erratic circuit

behavior. To avoid this problem, the pin is internal clamped

slightly below –300 mV which means that if more current is

injected before reaching the ESD forward drop, then the

maximum peak reduction is kept to 40%. If the voltage

finally forward biases the internal Zener diode, then care

must be taken to avoid injecting a current beyond –2 mA.

Given the value of ROPPU, there is no risk in the present

example. Finally, please note that another comparator

internally fixes the maximum peak current set point to 0.8 V

even if the OPP pin is adversely biased above 0 V.

Frequency Foldback

The reduction of no−load standby power associated with

the need for improving the efficiency, requires a change in

the traditional fixed−frequency type of operation. This

controller implements a switching frequency foldback when

the feedback voltage passes below a certain level, Vfold, set

around 1.9 V. At this point, the oscillator turns into a

Voltage−Controlled Oscillator and reduces its switching

frequency. Below this point, the frequency no longer

changes and the feedback level still controls the peak current

setpoint. When the feedback voltage reaches 1 V, the peak

current freezes to (250 mV or »31% of the maximum 0.8−V

setpoint). If the power continues to decrease, the part enters

skip cycle at a moderate peak current for the best noise−free

performance in no−load conditions. Figure 8 depicts the

adopted scheme for the part.

Frequency

Peak current setpoint

Fsw

130 kHz

FB

65 kHz

VCS

max

0.8 V

max

26 kHz

min skip

VFB

400 mV 1.9 V 4 V

1.5 V 3.2 V

Vfold,end Vfold

≈0.47 V

min

≈0.25 V

Vskip Vfreeze

0.4 V 1 V

VFB

Vfold 3.2 V

1.9 V

Figure 8. By observing the voltage on the feedback pin, the controller reduces its

switching frequency for an improved performance at light load.

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